Natural gas typically consists of low hydrocarbons such as methane and some level of acidic impurities. Untreated natural gas is commonly referred to as sour gas. Before being able to use the natural gas, the acidic impurities need to be removed. This is commonly known as sweetening. Typical impurities are CO2, H2S, mercaptans (R—SH), carbonyl sulphide and/or carbon disulphide.
In the art, gases, other than untreated natural gas, comprising H2S and COS are sometimes also referred to as “sour gas.” This concerns, in addition to natural gas, synthesis gas, gas produced by coal gasification, or fumes from a combustion process. In the present description, the term “sour gas” generally indicates a gas comprising H2S and COS, and requiring deacification, and more particularly indicates, preferably, untreated natural gas.
Known methods for removing acidic impurities employ absorption in amine solutions. However, this method is not suitable for the removal of carbonyl sulphide (O═C═S; mostly known as COS) and carbonyl disulphide (mostly known as CS2). Regulatory changes require a further reduction of all sulfur containing compounds including COS.
A known method for removing COS and CS2 consists of a two step process, where in a first step, the COS and CS2 is converted into H2S. In a second reactor the H2S is removed. In another method, in a first reactor the COS and CS2 are hydrolyzed and in a second reactor again the H2S is removed. A disadvantage of the latter method is that the absorption liquid does not efficiently absorb H2S, COS and CS2 at the same time. Particularly in the event that relatively high levels of H2S are present (which is typical for sour gas), the removal of COS and CS2 can be insufficient.
A method that has been proposed to solve this bottleneck, involves the heterogenic, catalytic conversion of contaminants including COS and CS2 into hydrogen sulphide in a separate hydrogenation reactor operating in the gas phase. As a catalyst, typically heavy metals such as iron, lead, molybdenum, nickel, tungsten or sulphide of these metals are employed. A disadvantage of this method is that high operating temperature is needed (200° C.-300° C.).
Therefore, it has been suggested in the alternative to treat gases at such high temperature with basic reacting salts of inorganic or organic bases or any mixtures of them, dissolved in water or alcohol, i.e. with a basic scrubbing medium, so that the contaminants COS and CS2 are changed by means of hydrolysis into hydrogen sulphide according to the equilibrium reactions: (COS+H2OCO2+H2S and CS2+2HCO2+2H2S). The hydrogen sulphide can in turn be removed together with the host gas from the reaction zone. In that practice, the hydrolysis reaction is limited by the equilibrium constant. When the partial pressure of H2S reaches the equilibrium a relatively high level of COS and CS2 remains in the emissions.
Further, another problem is incurred in the event that a basic absorbing liquid is used. For, the basicity of the absorbing medium has to be maintained in spite of the presence of H2S and CO2. These compounds are to be converted into sulphides and carbonates thus reducing the basicity of the material as a whole. The hydrogen sulphide that is present as well as the hydrogen sulphide that is formed catalytically will then have to be removed from the gas in a separate step.
Some background art addresses the treatment of gas, other than natural gas, in order to deacidify such gases that contain H2S and COS, or at least remove COS therefrom. E.g., WO 00/35806 concerns the removal of COS from synthesis gas, in particular synthesis gas produced by gasification of coal. Such gas is essentially different from natural gas. In particular, the process in WO 00/35806 relates to the removal of COS from gasified coal or mixtures containing coal. In this gasification process the synthesis gas that is produced includes particulate matter such as coal ash, which is later used as the catalyst. The untreated natural gas does not contain this particulate matter as it consists typically of low hydrocarbons such as methane.
It is desired to improve the removal of COS and/or CS2 from sour gas, preferably from natural gas, particularly untreated natural gas, and more particularly to provide a process that is capable of removing COS and/or CS2 more efficiently, yet avoiding the high temperatures required in the art.